Adjustable time duration for driving pulse-width modulation (PWM) output to reduce thermal noise

ABSTRACT

Noise introduced in an output signal of a pulse-width modulator (PWM) may be reduced by changing the time duration that a switch is driving the output node. Because the power supplies coupled to the switches are the source of noise in the output signal of the PWM, the time duration that the power supplies are driving the output may be reduced to obtain a subsequent reduction in noise in the output signal. For example, when a small signal is desired to be output by the PWM, the switches may be operated for shorter time durations. Thus, the switches couple the noise sources to ground for a duration of a cycle to reduce contribution of noise to the output. But, when a larger signal is desired to be output by the PWM, the switches may be operated for longer time durations or the conventional time durations described above.

FIELD OF THE DISCLOSURE

This application is a continuation of U.S. patent application Ser. No.14/971,109 to Melanson et al. filed on Dec. 16, 2015 and entitled“Adjustable Time Duration for Driving Pulse-Width Modulation (PWM)Output to Reduce Thermal Noise,” which is incorporated by reference inits entirety.

FIELD OF THE DISCLOSURE

The instant disclosure relates to signal modulation. More specifically,portions of this disclosure relate to pulse-width modulation (PWM) withreduced noise.

BACKGROUND

Information can be represented as signals in numerous manners. One suchmanner is modulation, in which information is encoded by varying theproperties of a carrier signal. One example of modulation is amplitudemodulation (AM) employed in AM radios. In an AM radio signal, anamplitude of a signal at a fixed frequency is varied over time accordingto the information to be conveyed. Another example of modulation ispulse width modulation (PWM). Pulse-width modulation (PWM) is frequentlyused in electronic devices and telecommunications, such as mobilephones, to transmit information from one point to another point, whetherthat is across a circuit board or across miles of airwaves. Pulse-widthmodulation (PWM) encodes data as a series of pulses in a pulsing signal.The pulses are generated by a circuit, such as the one illustrated inFIG. 1.

FIG. 1 is a circuit schematic showing a conventional pulse-widthmodulator. A modulator 110 may include switches 116 and 118 coupled topositive and negative power supplies, respectively. The switches 116 and118 may be configured to drive an output node 102 when coupled to node112 and not drive the output node 102 when coupled to node 114. Controlsignals CTRL1 and CTRL2 supplied to switches 116 and 118 may couple theswitches 116 and 118 to node 112 to generate pulses at the output node102. For example, an average zero output may be generated by applyingthe control signals illustrated in FIG. 2A to the switches 116 and 118of FIG. 1. A first control CTRL1 signal 202 and a second control CTRL2signal 204 may be high at time 212 to couple the switch 116 to node 112and the switch 118 to node 114. At time 214, the switches 116 and 118reverse such that switch 116 is coupled to node 114 and switch 118 iscoupled to node 112. As another example, an average positive output maybe generated by applying the control signals illustrated in FIG. 2B tothe switches 116 and 118 of FIG. 1. A first control CTRL1 signal 206 maygo high at time 212, followed by a second control CTRL2 signal 208 attime 216. Now, the output at output node 102 is an average of thestrength of the drive from switches 116 and 118 but weighted by theamount of time that each of the switches is coupled to the node 112.When the drive from switches 116 and 118 is equal, the output at outputnode 102 is a positive pulse between time 212 and time 214, because thefirst control CTRL1 signal causes the switch 116 to be coupled to node112 for a longer duration that control CTRL2 signal causes the switch118 to be coupled to node 112.

In the pulse-width modulation (PWM) technique described above withreference to FIG. 1, the output at output node 102 is not a perfectsignal. Noise is generated by the power supplies for the switches 116and 118, and that noise becomes part of the output at output node 102.One type of noise introduced by the switches 116 and 118 is thermalnoise. Thermal noise is variations in the driving of the node 112 thatoccur as a result of thermal fluctuations within the power sources forthe switches 116 and 118. The thermal fluctuations may be due toenvironmental changes or heat produced by operating the power sources.In conventional pulse-width modulation (PWM) techniques described above,thermal noise is always present in the output signal because theswitches 112 and 118 are never both coupled to node 114. Instead, one ofthe switches 116 and 118 is always driving the output node 102.

As electronics continue to advance, the total power consumed by thedevices is reduced resulting in large problems with noise. The powerconsumption reduction is often due to a decrease of the magnitude of thesupply voltages of the electronics necessitated by a shrink of thetransistors that are the building blocks of the electronics. The sourcesof noise, such as thermal noise in power supplies, does not necessarilydecrease along with the power consumption. Thus, the relativecontribution of noise to the output signal increases as powerconsumption decreases. Because many applications require a sufficientsignal-to-noise ratio (SNR), the noise introduced by the power sourcesis a problem when a signal level is small.

Shortcomings mentioned here are only representative and are includedsimply to highlight that a need exists for improved electricalcomponents, particularly for pulse-width modulators employed inconsumer-level devices, such as mobile phones. Embodiments describedherein address certain shortcomings but not necessarily each and everyone described here or known in the art.

SUMMARY

Noise introduced in an output signal of a pulse-width modulator (PWM)may be reduced by changing the time duration that a switch is drivingthe output node. Because the power supplies coupled to the switches arethe source of noise in the output signal of the PWM, the time durationthat the power supplies are driving the output may be reduced to obtaina subsequent reduction in noise in the output signal. For example, whena small signal is desired to be output by the PWM, the switches may beoperated for shorter time durations. Thus, the switches couple the noisesources to ground for a duration of a cycle to reduce contribution ofnoise to the output. This switching technique may be implemented in oneembodiment as a duty cycle of less than fifty percent for the switches.But, when a larger signal is desired to be output by the PWM, theswitches may be operated for longer time durations or the conventionaltime durations described above.

According to one embodiment, an apparatus may include an output node; afirst source for driving the output node in a positive direction; afirst switch coupled to the first source and to the output node, inwhich the first switch is configured such that when connected in a firstphase drives the output node in a positive direction and contributesnoise and when connected in a second phase does not contribute to outputnoise or drive the output node in any direction; a second source fordriving the output node in a negative direction; a second switch coupledto the second source and to the output node, in which the second switchis configured such that when connected in a third phase drives theoutput node in a negative direction and contributes noise and whenconnected in a fourth phase does not contribute to output noise or drivethe output node in any direction; and/or a controller coupled to thefirst switch and coupled to the second switch. The controller may beconfigured to perform steps including receiving a reference signal;operating the first switch and the second switch to generate a pulsewidth modulation (PWM) representation of the reference signal at theoutput node; and/or adjusting a time duration that the first switch andthe second switch are coupled to the output node based, at least inpart, on an envelope level of a desired output signal at the outputnode.

In certain embodiments, the controller may be configured to performsteps including adjusting the time duration that the first switch andthe second switch are coupled to the output node proportional to theenvelope level; in a first mode, coupling the first switch and thesecond switch to the output node for a first duration of time; in asecond mode, coupling the first switch and the second switch to theoutput node for a second duration of time shorter than the firstduration of time; switching between the first mode and the second modebased, at least in part, on the envelope level; switching from the firstmode to the second mode when the envelope level is at least 10 decibelsbelow full scale; adjusting the time duration based, at least in part,on an amplitude of the reference signal; adjusting the time durationbased, at least in part, on a desired volume level; operating the firstswitch for a longer duration of time than the second switch to generatea pulse; and/or coupling the first switch and the second switch to theoutput node with a duty cycle below fifty percent.

In certain embodiments, the first source may be stronger than the secondsource, and the controller may be configured to operate the first switchfor a longer duration of time than the second switch to generate apositive pulse at the output node.

According to another embodiment, a method may include receiving, by acontroller, a reference signal; operating, by the controller, a firstswitch and a second switch of a pulse width modulator to generate apulse width modulation (PWM) representation of the reference signal atan output node, wherein the first switch is operated in a first phase todrive an output node in a positive direction and contribute noise and ina second phase to not contribute to output noise or drive the outputnode in any direction, and wherein the second switch is operated in athird phase to drive the output node in a negative direction andcontribute noise and in a fourth phase to not contribute to output noiseor drive the output node in any direction; and/or adjusting, by thecontroller, a time duration that the first switch and the second switchare coupled to the output node based, at least in part, on an envelopelevel of a desired output signal.

In certain embodiments, the step of adjusting the time duration mayinclude adjusting the time duration proportional to the envelope level;the step of adjusting the time duration may include for a first range ofthe envelope level, coupling the first switch and the second switch tothe output node for a first duration of time; the step of adjusting thetime duration may include for a second range of the envelope level,coupling the first switch and the second switch to the output node for asecond duration of time shorter than the first duration of time; thestep of adjusting the time duration comprises switching from the firstduration of time to the second duration of time when the envelope levelis at least 10 decibels below full scale; the step of adjusting the timeduration may be based, at least in part, on an amplitude of thereference signal; and/or the step of adjusting the time duration may bebased, at least in part, on a desired gain level.

According to another embodiment, an apparatus may include a pulse widthmodulation controller configured to control a pulse width modulator atleast by switching a first switch to and from an output node and byswitching a second switch to and from the output node to generate apulse width modulation (PWM) representation of a reference signal at theoutput node, wherein the first switch is operated in a first phase todrive the output node in a positive direction and contribute noise andin a second phase to not contribute to output noise or drive the outputnode in any direction, wherein the second switch is operated in a thirdphase to drive the output node in a negative direction and contributenoise and in a fourth phase to not contribute to output noise or drivethe output node in any direction, and wherein the pulse width modulationcontroller is configured to adjust a time duration within a cycle thatthe first switch and the second switch are both coupled to the outputnode based, at least in part, on an envelope level of a desired outputsignal at the output node.

In certain embodiments, the controller may be configured to adjust thetime duration proportional to the envelope level; the controller isconfigured to adjust the time duration by, for a first range of theenvelope level, coupling the first switch and the second switch to theoutput node for a first duration of time; the controller may beconfigured to adjust the time duration by, for a second range of theenvelope level, coupling the first switch and the second switch to theoutput node for a second duration of time shorter than the firstduration of time; the controller may be configured to switch from thefirst duration of time to the second duration of time when the envelopelevel is at least 10 decibels below full scale;

The foregoing has outlined rather broadly certain features and technicaladvantages of embodiments of the present invention in order that thedetailed description that follows may be better understood. Additionalfeatures and advantages will be described hereinafter that form thesubject of the claims of the invention. It should be appreciated bythose having ordinary skill in the art that the conception and specificembodiment disclosed may be readily utilized as a basis for modifying ordesigning other structures for carrying out the same or similarpurposes. It should also be realized by those having ordinary skill inthe art that such equivalent constructions do not depart from the spiritand scope of the invention as set forth in the appended claims.Additional features will be better understood from the followingdescription when considered in connection with the accompanying figures.It is to be expressly understood, however, that each of the figures isprovided for the purpose of illustration and description only and is notintended to limit the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the disclosed system and methods,reference is now made to the following descriptions taken in conjunctionwith the accompanying drawings.

FIG. 1 is a circuit schematic showing a conventional pulse-widthmodulator.

FIG. 2A are graphs illustrating control signals for operating thecircuit of FIG. 1 to generate a zero output.

FIG. 2B are graphs illustrating control signals for operating thecircuit of FIG. 1 to generate a positive output.

FIG. 3A are graphs illustrating control signals for operating apulse-width modulator with reduced noise to generate a zero outputaccording to one embodiment of the disclosure.

FIG. 3B are graphs illustrating control signals for operating apulse-width modulator with reduced noise to generate a positive outputaccording to one embodiment of the disclosure.

FIG. 4 is a flow chart illustrating operation of a pulse-width modulatorto adjust a time duration that switches are active according to oneembodiment of the disclosure.

FIG. 5 is a circuit schematic illustrating a pulse-width modulatoroperated by a controller to adjust time duration that switches areactive according to one embodiment of the disclosure.

FIG. 6 is a flow chart illustrating a method of operating switches of apulse-width modulator by a controller to reduce noise according to oneembodiment of the disclosure.

FIG. 7 is a flow chart illustrating a method of operating switches of apulse-width modulator by a controller using predefined thresholds toreduce noise according to one embodiment of the disclosure.

DETAILED DESCRIPTION

Noise in an output signal of a pulse-width modulator (PWM) may bereduced by decreasing the amount of time the sources of the noise arecoupled to the output node of the pulse-width modulator. For example,the time that switches are switched on to couple power sources to theoutput node may be reduced in comparison to the conventional techniques,resulting in a corresponding decrease of thermal noise contributed tothe output signal of the PWM. Operation of the switches may becontrolled by the control signals illustrated in FIG. 3A.

FIG. 3A are graphs illustrating control signals for operating apulse-width modulator with reduced noise to generate a zero outputaccording to one embodiment of the disclosure. A first control CTRL1signal 302, along with a second control CTRL2 signal 304, may go high attime 312 to switch switches that couple the power source to the outputnode. The control signals CTRL1 and CTRL2 may go low at time 314 toswitch the power sources coupled to the output node. A time duration 322during which the power sources are coupled to the output node of thepulse-width modulator (PWM) is shorter in time than a time duration 324between corresponding times 212 and 214 of the prior art.

Likewise, FIG. 3B are graphs illustrating control signals for operatinga pulse-width modulator with reduced noise to generate a positive outputaccording to one embodiment of the disclosure. A first control CTRL1signal 306 may go high at time 312 to switch on switches that couple thepower source to the output node. A second control CTRL2 signal 308 maygo high at time 316 after time 312. Then, the control CTRL1 and CTRL2signals 306 and 308 may go low at time 314. As with the control signalsof FIG. 3A, the time duration 322 and time duration 326 that theswitches couple the power supplies to the output node is reduced fromthe time duration 324 of the prior art. Thus, a reduction in noise atthe output of the pulse-width modulator (PWM) may be achieved.

The time duration 322 of the control pulses for operating switches mayvary based on a reference input signal that is being converted to apulse-width modulation representation. That is, the time duration may bebased on a desired output level for the pulse-width modulator that isindicated by the reference input signal. If a larger input signal isdetected and a larger output thus desired, the time duration may beincreased; if a smaller input signal is detected and a smaller outputthus desired, the time duration may be decreased. Smaller and larger mayrefer to an envelope level of the input or output signal. The desiredenvelope level may be dictated by factors such as a desired volume or anamplitude of the reference input signal. Control of a pulse-widthmodulator (PWM) may be performed according to the method illustrated inFIG. 4.

FIG. 4 is a flow chart illustrating operation of a pulse-width modulatorto adjust a time duration that switches are active according to oneembodiment of the disclosure. A method 400 begins at block 402 withreceiving a reference input signal for conversion to a pulse-widthmodulated output signal that is representative of the reference inputsignal. Then, at block 404, a first switch and a second switch of thepulse-width modulator may be operated based on the reference inputsignal. At block 406, a time duration that the first switch and thesecond switch are coupled to the output node may be adjusted based on anenvelope level of a desired output signal at the output of thepulse-width modulator. The step of block 406 may occur in real time withthe operation of the switches in block 404, such that the time durationis dynamically changing in response to the continued input received asthe reference input signal.

The method of FIG. 4 may be implemented in a pulse-width modulator, inone embodiment, in a controller configured to operate switches of thepulse-width modulator, such as the controller illustrated in FIG. 5.FIG. 5 is a circuit schematic illustrating a pulse-width modulatoroperated by a controller to adjust time duration that switches areactive according to one embodiment of the disclosure. A portion of apulse-width modulator (PWM) 500 may include switches 516 and 518configured to couple power supplies 512 and 514 to nodes 532 and 534,respectively. A switch 516 may toggle between coupling the power supply512 to the node 532 to drive an output at output node 502 in a positivedirection and coupling the power supply 512 to ground. A switch 518 maytoggle between coupling the power supply 514 to the node 534 to drive anoutput node 502 in a negative direction and coupling the power supply514 to ground.

A controller 522 may be configured to operate the switches 516 and 518by outputting control signals CTRL1 and CTRL2 that toggle the switches516 and 518. The controller 522 may be coupled to an input node 504 forreceiving a reference input signal for conversion to a pulse-widthrepresentation at output node 502. Although only one set of switches 516and 518 and corresponding power sources 512 and 514 is illustrated inFIG. 5, additional sets of switches and power sources may be included.For example, a finite impulse response (FIR) filter may include many ofthe sets of switches 516 and 518, such as between 8 and 128. The powersources 512 and 514 may be, for example, current sources that drivecurrent to a node selected by the switches 516 and 518.

The controller 522 may generate control CTRL1 and CTRL2 signals withvarious time durations selected according to a desired envelope level ofan output signal generated at output node 502. The controller 522 mayimplement shorter time durations by generating control CTRL1 and CTRL2signals similar to those shown in FIG. 3A and FIG. 3B. The controller522 may implement longer time durations by generating control CTRL1 andCTRL2 signals similar to those shown in FIG. 2A and FIG. 2B. The shortertime duration and longer timer duration may be switched according to analgorithm executed by the controller 522. One such algorithm isillustrated in FIG. 6.

FIG. 6 is a flow chart illustrating a method of operating switches of apulse-width modulator by a controller to reduce noise according to oneembodiment of the disclosure. A method 600 begins at block 602 with thecontroller receiving a reference input signal for conversion to apulse-width modulation representation. At block 604, the controllerdetermines whether the reference input signal is a small signal or not asmall signal. A small signal, in one embodiment, may be a signal withsignal-to-noise ratio (SNR) of 20 decibels (dB) or less. A small signalin other embodiments, may be a signal with a signal-to-noise ratio (SNR)of 10 decibels (dB) or less or may be a signal with a SNR of 60 decibels(dB) or less. Alternatively or additionally, the block 604 may includedetermining whether an envelope level of a desired output signal is asmall signal. When a small signal is detected at block 604, the method600 proceeds to block 606 to operate switches of the pulse-widthmodulator at short time durations. For example, block 606 may beimplemented by the controller 522 operating the switches 516 and 518 ofFIG. 5 with the control CTRL1 and CTRL2 signals illustrated in FIG. 3Aand FIG. 3B. Referring back to FIG. 6, when a small signal is notdetected at block 604, the method 600 proceeds to block 608 to operateswitches of the pulse-width modulator at long time durations. Forexample, block 608 may be implemented by the controller 522 operatingthe switches 516 and 518 of FIG. 5 with the control CTRL1 and CTRL2signals illustrated in FIG. 2A and FIG. 2B. Although the signals of FIG.2A and FIG. 2B are mentioned, any time duration longer than the shorttime duration of block 606 may be implemented at block 608.

The time duration adjustment of FIG. 6 illustrates two time durations,however, a large number of time durations may be implemented by thecontroller 522, such as when executing the method illustrated in FIG. 7.FIG. 7 is a flow chart illustrating a method of operating switches of apulse-width modulator by a controller using predefined thresholds toreduce noise according to one embodiment of the disclosure. A method 700begins at block 702 with receiving a reference input signal forconversion to a pulse-width representation. At block 704, the controllerdetermines if the input signal is smaller than a first threshold. If so,the method 700 proceeds to block 706 to operate the switches using afirst time duration. If not, then the method 700 proceeds to block 708where the controller determines if the input signal is smaller than asecond larger threshold. If so, the method 700 proceeds to block 710 tooperate the switches using a second longer time duration. If not, thenthe method 700 proceeds to block 712 to operate the switches using athird still longer time duration that is longer than both the first andsecond time durations. Alternatively or additionally, the blocks 704 and708 may include determining whether a desired envelope level of anoutput signal is less than a first or second threshold.

Although thresholds are shown in FIG. 6 and FIG. 7 and used by thecontroller for adjusting the time duration of operating the switches,the time duration may also be set in a continuous or nearly continuousmanner. For example, the time duration may be set proportional to anamplitude or envelope level of the reference input signal. In thisembodiment, the time duration may be set based on a formula using theamplitude or envelope level of the reference input signal as a variable.

The schematic flow chart diagrams of FIG. 4, FIG. 6, and FIG. 7 aregenerally set forth as a logical flow chart diagram. As such, thedepicted order and labeled steps are indicative of aspects of thedisclosed method. Other steps and methods may be conceived that areequivalent in function, logic, or effect to one or more steps, orportions thereof, of the illustrated method. Additionally, the formatand symbols employed are provided to explain the logical steps of themethod and are understood not to limit the scope of the method. Althoughvarious arrow types and line types may be employed in the flow chartdiagram, they are understood not to limit the scope of the correspondingmethod. Indeed, some arrows or other connectors may be used to indicateonly the logical flow of the method. For instance, an arrow may indicatea waiting or monitoring period of unspecified duration betweenenumerated steps of the depicted method. Additionally, the order inwhich a particular method occurs may or may not strictly adhere to theorder of the corresponding steps shown.

If implemented in firmware and/or software, functions described abovemay be stored as one or more instructions or code on a computer-readablemedium. Examples include non-transitory computer-readable media encodedwith a data structure and computer-readable media encoded with acomputer program. Computer-readable media includes physical computerstorage media. A storage medium may be any available medium that can beaccessed by a computer. By way of example, and not limitation, suchcomputer-readable media can comprise random access memory (RAM),read-only memory (ROM), electrically-erasable programmable read-onlymemory (EEPROM), compact disc read-only memory (CD-ROM) or other opticaldisk storage, magnetic disk storage or other magnetic storage devices,or any other medium that can be used to store desired program code inthe form of instructions or data structures and that can be accessed bya computer. Disk and disc includes compact discs (CD), laser discs,optical discs, digital versatile discs (DVD), floppy disks and Blu-raydiscs. Generally, disks reproduce data magnetically, and discs reproducedata optically. Combinations of the above should also be included withinthe scope of computer-readable media.

In addition to storage on computer readable medium, instructions and/ordata may be provided as signals on transmission media included in acommunication apparatus. For example, a communication apparatus mayinclude a transceiver having signals indicative of instructions anddata. The instructions and data are configured to cause one or moreprocessors to implement the functions outlined in the claims.

Although the present disclosure and certain representative advantageshave been described in detail, it should be understood that variouschanges, substitutions and alterations can be made herein withoutdeparting from the spirit and scope of the disclosure as defined by theappended claims. Moreover, the scope of the present application is notintended to be limited to the particular embodiments of the process,machine, manufacture, composition of matter, means, methods and stepsdescribed in the specification. For example, although “high” and “low”signals are described, these are only relative terms and thus theembodiments disclosed herein may be easily modified to instead operatewhen signals “go low” or “go high.” As one of ordinary skill in the artwill readily appreciate from the present disclosure, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized. Accordingly,the appended claims are intended to include within their scope suchprocesses, machines, manufacture, compositions of matter, means,methods, or steps.

What is claimed is:
 1. An apparatus, comprising: a controller coupled toa plurality of switches for generating a pulse-width modulation signalat an output node, and wherein the controller is configured to: receivean input signal for conversion to a pulse width modulationrepresentation; determine whether the input signal is a small signal ofdetermining whether a signal-to-noise ratio (SNR) of the input signal isbelow a threshold level; when the input signal is a small signal,operate the plurality of switches at short time durations having a firsttime duration; and when the input signal is not a small signal, operatethe plurality of switches at long time durations having a second timeduration that is longer than the first time duration.
 2. The apparatusof claim 1, wherein the controller is configured to operate theplurality of switches at short time durations by operating at least oneof the plurality of switches with a duty cycle of less than fiftypercent.
 3. The apparatus of claim 1, wherein the controller isconfigured to generate the pulse-width modulation signal at the outputnode by operating a first switch and a second switch of the plurality ofswitches to generate a pulse width modulation (PWM) representation ofthe input signal at the output node.
 4. The apparatus of claim 3,wherein the controller is further configured to: adjust a time durationthat the first switch and the second switch are coupled to the outputnode based, at least in part, on an envelope level of a desired outputsignal at the output node by: in a first mode, coupling the first switchand the second switch to the output node for the first time duration; ina second mode, coupling the first switch and the second switch to theoutput node for the second time duration; and switching between thefirst mode and the second mode based, at least in part, on the envelopelevel.
 5. The apparatus of claim 4, wherein the controller is configuredto adjust the time duration that the first switch is in the first phaseand that the second switch is in the third phase such that the timeduration is shorter at lower desired output signal levels.
 6. Theapparatus of claim 3, wherein the first switch is configured such thatwhen connected in a first phase drives the output node in a positivedirection and contributes noise and when connected in a second phasedoes not contribute to output noise or drive the output node in anydirection, and wherein the second switch is configured such that whenconnected in a third phase drives the output node in a negativedirection and contributes noise and when connected in a fourth phasedoes not contribute to output noise or drive the output node in anydirection.
 7. The apparatus of claim 3, wherein the controller isconfigured to adjust the time duration based, at least in part, on adesired volume level.
 8. The apparatus of claim 1, wherein thecontroller is configured to determine whether the input signal is asmall signal by determining whether the input signal is below 10decibels (dB).
 9. The apparatus of claim 1, wherein the controller isconfigured to determine whether the input signal is a small signal bydetermining whether the input signal is below 20 decibels (dB).
 10. Theapparatus of claim 1, wherein the controller is configured to determinewhether the input signal is a small signal by determining whether theinput signal is below 60 decibels (dB).
 11. A method, comprising:receiving an input signal for conversion to a pulse width modulationrepresentation at an output node; determining whether the input signalis a small signal by determining whether a signal-to-noise ratio (SNR)of the input signal is below a threshold level; when the input signal isa small signal, operating the plurality of switches at short timedurations having a first time duration; and when the input signal is nota small signal, operating the plurality of switches at long timedurations having a second time duration that is longer than the firsttime duration.
 12. The method of claim 11, wherein the step of operatingthe plurality of switches at short time durations comprises operating atleast one of the plurality of switches with a duty cycle of less thanfifty percent.
 13. The method of claim 11, wherein the step ofgenerating the pulse-width modulation signal comprises operating a firstswitch and a second switch of the plurality of switches to generate apulse width modulation (PWM) representation of the input signal at theoutput node.
 14. The method of claim 13, adjusting a time duration thatthe first switch and the second switch are coupled to the output nodebased, at least in part, on an envelope level of a desired output signalat the output node, wherein adjusting comprises: in a first mode,coupling the first switch and the second switch to the output node forthe first time duration; in a second mode, coupling the first switch andthe second switch to the output node for the second time duration; andswitching between the first mode and the second mode based, at least inpart, on the envelope level.
 15. The method of claim 14, furthercomprising adjusting the time duration based, at least in part, on adesired volume level.
 16. The method of claim 13, wherein the firstswitch is configured such that when connected in a first phase drivesthe output node in a positive direction and contributes noise and whenconnected in a second phase does not contribute to output noise or drivethe output node in any direction, and wherein the second switch isconfigured such that when connected in a third phase drives the outputnode in a negative direction and contributes noise and when connected ina fourth phase does not contribute to output noise or drive the outputnode in any direction.
 17. The method of claim 16, further comprisingadjusting the time duration that the first switch is in the first phaseand that the second switch is in the third phase such that the timeduration is shorter at a lower desired output signal.
 18. The method ofclaim 11, wherein the step of determining whether the input signal is asmall signal comprises determining whether the input signal is below 10decibels (dB).
 19. The method of claim 11, wherein the step ofdetermining whether the input signal is a small signal comprisesdetermining whether the input signal is below 20 decibels (dB).
 20. Themethod of claim 11, wherein the step of determining whether the inputsignal is a small signal comprises determining whether the input signalis below 60 decibels (dB).